1. Field of the Invention
This invention relates to overbased calcium sulfonate greases made with delayed addition of non-aqueous converting agents and the method for manufacturing such greases to provide improvements in both thickener yield and expected high temperature utility as demonstrated by dropping point, even when the oil-soluble overbased calcium sulfonate used to make the grease is considered to be of poor quality.
2. Description of Related Art
Overbased calcium sulfonate greases have been an established grease category for many years. One known process for making such greases is a two-step process involving the steps of “promotion” and “conversion.”Typically the first step (“promotion”) is to react a stoichiometric excess amount of calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) as the base source with an alkyl benzene sulfonic acid, carbon dioxide (CO2), and with other components to produce an oil-soluble overbased calcium sulfonate with amorphous calcium carbonate dispersed therein. These overbased oil-soluble calcium sulfonates are typically clear and bright and have Newtonian rheology. In some cases, they may be slightly turbid, but such variations do not prevent their use in preparing overbased calcium sulfonate greases. For the purposes of this disclosure, the terms “overbased oil-soluble calcium sulfonate” and “oil-soluble overbased calcium sulfonate” and “overbased calcium sulfonate” refer to any overbased calcium sulfonate suitable for making calcium sulfonate greases. Typically the second step (“conversion”) is to add a converting agent or agents, such as propylene glycol, iso-propyl alcohol, water, formic acid or acetic acid, to the product of the promotion step, along with a suitable base oil (such as mineral oil) if needed to keep the initial grease from being too hard, to convert the amorphous calcium carbonate to a very finely divided dispersion of crystalline calcium carbonate. When acetic acid or other acids are used as a converting agent, typically water and another non-aqueous converting agent (a third converting agent, such as an alcohol) are also used; alternatively only water (without the third converting agent) is added, but the conversion then typically occurs in a pressurized vessel. Because an excess of calcium hydroxide or calcium oxide is used to achieve overbasing, a small amount of residual calcium oxide or calcium hydroxide may also be present as part of the oil soluble overbased calcium sulfonate and will be dispersed in the initial grease structure. The crystalline form of the calcium carbonate is preferably calcite. This extremely finely divided calcium carbonate, also known as a colloidal dispersion, interacts with the calcium sulfonate to form a grease-like consistency. Such overbased calcium sulfonate greases produced through the two-step process have come to be known as “simple calcium sulfonate greases” and are disclosed, for example, in U.S. Pat. Nos. 3,242,079; 3,372,115; 3,376,222, 3,377,283; and 3,492,231.
It is also known in the prior art to combine these two steps, by carefully controlling the reaction, into a single step. In this one-step process, the simple calcium sulfonate grease is prepared by reaction of an appropriate sulfonic acid with either calcium hydroxide or calcium oxide in the presence of carbon dioxide and a system of reagents that simultaneously act as both promoter (creating the amorphous calcium carbonate overbasing by reaction of carbon dioxide with an excess amount of calcium oxide or calcium hydroxide) and converting agents (converting the amorphous calcium carbonate to very finely divided crystalline calcium carbonate). Thus, the grease-like consistency is formed in a single step wherein the overbased, oil-soluble calcium sulfonate (the product of the first step in the two-step process) is never actually formed and isolated as a separate product. This one-step process is disclosed, for example, in U.S. Pat. Nos. 3,661,622; 3,671,012; 3,746,643; and 3,816,310.
In addition to simple calcium sulfonate greases, calcium sulfonate complex greases are also known in the prior art. These complex greases are typically produced by adding a strong calcium-containing base, such as calcium hydroxide or calcium oxide, to the simple calcium sulfonate grease produced by either the two-step or one-step process and reacting with up to stoichiometrically equivalent amounts of complexing acids, such as 12-hydroxystearic acid, boric acid, acetic acid (which may also be a converting agent when added pre-conversion), or phosphoric acid. The claimed advantages of the calcium sulfonate complex grease over the simple grease include reduced tackiness, improved pumpability, and improved high temperature utility. Calcium sulfonate complex greases are disclosed, for example, in U.S. Pat. Nos. 4,560,489; 5,126,062; 5,308,514; and 5,338,467.
Much of the known prior art using the two step method teaches the addition of all converting agents (water and non-aqueous converting agents) at the same time and usually prior to heating. However, a few prior art references disclose a time interval (although always poorly defined or not defined at all) between the addition of water and the addition of at least part of the non-aqueous converting agent(s). For example, U.S. Pat. No. 4,560,489 discloses a process (examples 1-3) where base oil and overbased calcium carbonate are heated to around 150° F., then water is added, the mixture is then heated to around 190° F. before adding acetic acid and methyl Cellosolve (a highly toxic monomethylether of ethylene glycol). The resulting grease contains greater than 38% overbased calcium sulfonate and the '489 patent points out that the ideal amount of overbased calcium sulfonate for the processes disclosed therein is around 41-45%, since according to the '489 patent using less than 38% results in a soft grease. The resulting grease of example 1 in the '489 patent has a dropping point of around only 570° F. The '489 patent does not state the duration of delay between the addition of water and the addition of the non-aqueous converting agents, but indicates that the addition was immediate after a period of heating from 150 F to just 190 F. The dropping point and thickener yield in the '489 patent are not desirable.
Additionally, U.S. Pat. Nos. 5,338,467 and 5,308,514 disclose the use of a fatty acid, such as 12-hydroxystearic acid, as a converting agent used along with acetic acid and methanol, where there is no delay for the addition of the fatty acid but some interval between the addition of water and the addition of acetic acid and methanol. Example B in the '514 patent and example 1 in the '467 patent both describe a process where water and the fatty acid converting agent are added to other ingredients (including the overbased calcium sulfonate and base oil), then heated to around 140-145° F. before adding acetic acid followed by methanol. The mixture is then heated to around 150-160° F. until conversion is complete. The amount of overbased calcium sulfonate in the final grease products in both examples is 32.2, which is higher than desirable. These patents do not state the duration of delay between the addition of water and fatty acid and the addition of the acetic acid and methanol, but indicates that the addition was immediate after an unspecified period of heating. Similar processes are disclosed in example A of the '467 patent and example C of the '514 patent except all of the fatty acid was added post conversion, so the only non-aqueous converting agents used were the acetic acid and methanol added after the mixture with water was heated to 140-145 F. The amount of overbased calcium sulfonate in these examples is even higher than the previous examples at 40%. In addition to not achieving ideal thickener yield results, all these processes use methanol as a converting agent, which has environmental drawbacks. The use of volatile alcohols as converting agents may result in venting these ingredients to the atmosphere as a later part of the grease-making process, which is prohibited in many parts of the world. If not vented, the alcohols must be recovered by water scrubbing or water traps, which results in hazardous material disposal costs. As such, there is a need for a process that achieves better thickener yields, preferably without requiring the use of volatile alcohols as converting agents.
Better thickener yields are achieved in example 10 of the '514 patent, but the use of excess lime is taught as a requirement to achieve those results. In that example, water and excess lime are added together with other ingredients, the mixture is heated to 180-190 F while slowly adding acetic acid during the heating period. The resulting grease contained 23% overbased calcium sulfonate. While this thickener yield is better than others, there is still room for greater improvement without requiring the use of excess lime, which the '514 patent teaches as a requirement.
The other examples in '514 and '467 patents where there are thickener yields of 23% or less either involve the use of a pressurized kettle during conversion or are like the much greater part of the other prior art where there is no “delay” between the addition of water and the non-aqueous converting agents or both. These examples involve adding water and a fatty acid converting agent, mixing for 10 minutes without heating, and then adding acetic acid, either in a pressurized kettle or without pressure. Neither of these patents recognizes any benefit or advantage to the 10 minute interval for adding acetic acid, or the other heating delays in the examples discussed above, rather these patents focus the use of a fatty acid as a converting agent and the benefits of adding the fatty acid pre-conversion, post-conversion, or both as the reason for any observed yield improvements. Additionally, as discussed below, this 10 minute mixing interval without any heating is not a “delay” as that term is used herein, but is considered to be the same as adding the ingredients at the same time, recognizing that adding each ingredient takes at least some time and cannot occur instantaneously.
Additionally, the known prior art always teaches the use of calcium oxide or calcium hydroxide as the sources of basic calcium for production of calcium sulfonate greases or as a required component for reacting with complexing acids to form calcium sulfonate complex greases. The known prior art teaches that the addition of calcium hydroxide or calcium oxide needs to be in an amount sufficient (when added to the amount of calcium hydroxide or calcium oxide present in the overbased oil-soluble calcium sulfonate) to provide a total level of calcium hydroxide or calcium oxide sufficient to fully react with the complexing acids. As disclosed in co-pending U.S. application Ser. Nos. 13/664,574 and 13/664,768, the known prior art generally teaches that the presence of calcium carbonate (as a separate ingredient or as an “impurity” in the calcium hydroxide or calcium oxide, other than that presence of the amorphous calcium carbonate dispersed in the calcium sulfonate after carbonation), should be avoided for at least two reasons. The first being that calcium carbonate is generally considered to be a weak base, unsuitable for reacting with complexing acids to form optimum grease structures. The second being that the presence of unreacted solid calcium compounds (including calcium carbonate, calcium hydroxide or calcium oxide) interferes with the conversion process, resulting in inferior greases if the unreacted solids are not removed prior to conversion or before conversion is completed. However, Applicant has found that the addition of calcium carbonate as a separate ingredient (in addition to the amount of calcium carbonate contained in the overbased calcium sulfonate), calcium hydroxyapatite, or a combination thereof, either with or without added calcium hydroxide or calcium oxide, as ingredients for reacting with complexing acids produces a superior grease as described in the '574 and '768 applications.
There are a couple of prior art references that disclose the addition of crystalline calcium carbonate as a separate ingredient (in addition to the amount of calcium carbonate contained in the overbased calcium sulfonate), but those greases have poor thickener yield (as the prior art teaches) or require nano-sized particles of calcium carbonate. For example, U.S. Pat. No. 5,126,062 discloses the addition of 5-15% calcium carbonate as a separate ingredient in forming a complex grease, but also requires the addition of calcium hydroxide to react with complexing acids. The added calcium carbonate is not the sole added calcium containing base for reacting with complexing acids in the '062 patent. In fact, the added calcium carbonate is specifically not added as a basic reactant for reaction with complexing acids. Instead, added calcium hydroxide is required as the specific calcium-containing base for reaction with all the complexing acids. Additionally, the resulting NGLI No. 2 grease contains 36%-47.4% overbased calcium sulfonate, which is a substantial amount of this expensive ingredient. In another example, Chinese publication CN101993767, discloses the addition of nano-sized particles of calcium carbonate (sized between 5-300 nm) being added to the overbased calcium sulfonate, although the reference does not indicate that the nano-sized particles of calcium carbonate are added as a reactant, or the sole separately added calcium containing base, for reacting with complexing acids. The use of nano-sized particles would add to the thickening of the grease to keep it firm, much like the fine dispersion of crystalline calcium carbonate formed by converting the amorphous calcium carbonate contained within the overbased calcium sulfonate (which can be around 20 A to 5000 A or around 2 nm to 500 nm according to the '467 patent), but would also substantially increase the costs over larger sized particles of added calcium carbonate. This Chinese patent application greatly emphasizes the absolute necessity of the added calcium carbonate having a true nano particle size. As shown in the example greases according to the invention described in copending '574 application, superior greases may be formed by the addition of micron sized calcium carbonate without requiring the use of the very expensive nano-sized particles and when using added calcium carbonate as one of or the sole added calcium containing base for reacting with complexing acids.
There are also prior art references for using tricalcium phosphate as an additive in lubricating greases. For instance, U.S. Pat. Nos. 4,787,992; 4,830,767; 4,902,435; 4,904,399; 4,929,371 all teach using tricalcium phosphate as an additive for lubricating greases. However, it is believed that no prior art references teach the use of calcium hydroxyapatite, having the formula Ca5(PO4)3OH or a mathematically equivalent formula with a melting point of around 1100 C, as a calcium-containing base for reaction with acids to make lubricating greases, including calcium sulfonate-based greases. There are several prior art references assigned to Showa Shell Sekiyu in Japan, including U.S. Patent Application Publication No. 2009/0305920, that describe greases containing tricalcium phosphate, Ca3(PO4)2, and reference a “hydroxyapatite” having the formula [Ca3(PO4)2]3.Ca(OH)2 as a source of tricalcium phosphate. This reference to “hydroxyapatite” is disclosed as a mixture of tricalcium phosphate and calcium hydroxide, which is not the same as the calcium hydroxyapatite disclosed and claimed in the '768 application and herein having the formula Ca5(PO4)3OH or a mathematically equivalent formula with a melting point of around 1100 C. Despite the misleading nomenclature, calcium hydroxyapatite, tricalcium phosphate, and calcium hydroxide are each distinct chemical compounds with different chemical formulae, structures, and melting points. When mixed together, the two distinct crystalline compounds tricalcium phosphate (Ca3(PO4)2) and calcium hydroxide (Ca(OH)2) will not react with each other or otherwise produce the different crystalline compound calcium hydroxyapatite (Ca5(PO4)3OH). The melting point of tricalcium phosphate (having the formula Ca3(PO4)2) is 1670 C. Calcium hydroxide does not have a melting point, but instead loses a water molecule to form calcium oxide at 580 C. The calcium oxide thus formed has a melting point of 2580 C. Calcium hydroxyapatite (having the formula Ca5(PO4)3OH or a mathematically equivalent formula) has a melting point of around 1100 C. Therefore, regardless of how inaccurate the nomenclature may be, calcium hydroxyapatite is not the same chemical compound as tricalcium phosphate, and it is not a simple blend of tricalcium phosphate and calcium hydroxide.
Additionally, it is desirable to have a calcium sulfonate complex grease composition and method of manufacture that results in both improved thickener yield and dropping point. Many of the known prior art compositions require an amount of overbased calcium sulfonate of least 36% (by weight of the final grease product) to achieve a suitable grease in the NGLI No. 2 category with a demonstrated dropping point of at least 575 F. The overbased oil-soluble calcium sulfonate is one of the most expensive ingredients in making calcium sulfonate grease, therefore it is desirable to reduce the amount of this ingredient while still maintaining a desirable level of firmness in the final grease (thereby improving thickener yield). In order to achieve a substantial reduction in the amount of overbased calcium sulfonate used, many prior art references utilize a pressure reactor. It is desirable to have an overbased calcium sulfonate grease wherein the percentage of overbased oil-soluble calcium sulfonate is less than 36% and the dropping point is consistently 575 F or higher when the consistency is within an NLGI No. 2 grade (or the worked 60 stroke penetration of the grease is between 265 and 295), without requiring a pressure reactor. Higher dropping points are considered desirable since the dropping point is the first and most easily determined guide as to the high temperature utility limitations of a lubricating grease.